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 Post subject: how to seal a ceiling
PostPosted: Thu Oct 05, 2017 2:30 am 
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Joined: Thu Aug 03, 2017 6:25 am
Posts: 6
Location: Milano, Italy
Second topic I post on the forum, I'm so glad about the very useful advice that I got about my project in my previous post
viewtopic.php?f=1&t=21207
that I dare asking about another project's detail that I really don't know how to deal with.

I own a small space in Milano (Italy) which I'm treating for sound isolation.

I know the rules here but I won't be very detailed about the description of my build because I maily designed it by studying this very forum.. and by reading books which are often quoted here as a reference. Maybe I did plenty of mistakes:) but right now I have sort of an 'emergency' question related to a very specific aspect of the build:)

So, like in many old building in Europe (beg. nineteenth century) my room's ceiling was built by applying plaster to cane lath between wooden beams thus creating a false ceiling with no fiberglass nor foam whatsoever.
The 3 leaf behaviour formula at page 342 of Marshal Long Architectural Acoustic, with my specific estimated leaf superficial weights and distances, forecasts a 30 hz and 75 hz resonances that I frankly cannot live with and which made me decide to destroy this plaster and cane lath false ceiling (very hard work but in my very little experience with acoustics I saw it's better follow the mathematics than hoping that it works and than having to re-do it again..).

The room in a room that I'm building is a 2 layer 20 mm fire rated drywall (18,6 kg per square meter x 2) with Green Glue, screwed to decoupled wooden stud (60 cm between studs) walls and ceiling (the ceiling rests on the decoupled walls. All filled with fluffy insulation, all doubly sealed with backer rod and acoustic caulk. The room in a room is 15 to 20 cm from the building walls and 25 cm from the false ceiling.

My question is:

how should I treat the ceiling now? I'm not done yet with the false ceiling removal but I can see already that there's wood slats between the beams which support the concrete floor of my neighbour. I visited him and I saw that all the surface resting on my ceiling has been renovated with more concrete to allow for the passage of water pipes. My measures indicate that there's a first thick layer of gravel, sand and concrete (about 10-11 cm thick) and than another 6 cm concrete pouring that they did in more recent years on top of that.

So, do I need to make sure that the new wooden ceiling that I'll expose is perfectly sealed or can I be confident that the 16 cm heavy stuff which is on top of the wood is already air tight?

I know that with the typical double drywall wall the advice would be to remove the drywall from one side and place it in between studs than caulk to provide for mass continuity and a good seal, but I'm not sure whether this would be the case in my build where I have such big and compact mass already in place..

Is there a need to apply an air-tight waterproof bitumen membrane for example, or maybe applying some liquid silicone and stucco? or drywall and caulk?

I ask this because it would be a lot more work than just filling with isolation, but I'll take care of this if you professionals say so!

I thank you in advance for your help.


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PostPosted: Mon Oct 16, 2017 7:10 pm 
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Joined: Thu Aug 03, 2017 6:25 am
Posts: 6
Location: Milano, Italy
No answer, that's fine:) I'll throw in some more details anyway, hoping for some advice..
I'm finished destroying the false ceiling and I now see why three leaf partitions are more forgiving.. The wooden slats supporting my neighbour's floor are spaced about 1 cm each, the sand that they used to make the concrete slab tends to crumble if I put my fingers in between the slabs. I asked two architect friends, they know nothing about acoustics nor sealed partitions but they told me not to worry about it, that I can be pretty sure that there's a decent air seal in place with all that concrete and the tiled neighbour's floor. I'm not very confident about it but I got no response from any acoustic pros. I just filled all the 40 cm space (20 cm between the beams + 20 cm before my decoupled gypsum board partition) with fluffy fiberglass.
I now have another concern.. Where the beams are embedded into the brick wall I see that the seal is not perfect, of course after 100 years or so I should have expected that..
Should I fill the gaps there? If so, how? Will expanding foam be enough?
It's very hard for me to understand because the available resources mostly talk about either stud structural walls or recent builds whereas in Europe many buildings are super old. In my case old is good, as the brick walls are 60 cm thick.. but still the overall conditions of the base materials, after so many years, are rather bad..


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